(1) Field of the Invention
The present invention relates to a fuel cell, and particularly, to a fuel cell using a separate gas cooling method (SGC) allowing a reactant air and a cooling air to flow separately.
(2) Description of the Related Art
A fuel cell, which converts chemical energy of a fuel gas directly into electric energy, is expected to be an efficient electricity generator, and the study of phosphoric acid fuel cells (PAFCS), molten carbonate fuel cells (MCFCs), and solid oxide fuel cells (SOFCs) has been conducted actively.
The cell temperature rises during the operation due to the heat of reaction and it is necessary to cool the fuel cell for efficient electricity generation at an adequate temperature. Conventionally, cooling plates, each having a plurality of cooling air passages that extend in parallel in one direction, are interposed between cell units at regular intervals to form a cell stack, and the cooling air is supplied from the inlets of the cooling plates for heat exchange.
However, according to the above construction, the cooling air inlet is over-cooled when the low-temperature cooling air flows in, whereas a cooling air outlet is heated when the post-heat-exchange cooling air flows out. This produces a large temperature gradient between the cooling air inlet and outlet, and thus making it impossible to cool the fuel cell evenly. This results in an uneven cell temperature and hence causes deterioration of the cell characteristics deterioration.
To maintain an even cell temperature even, a modified cooling plate is proposed in Japanese Patent Publication No. 62-2430. As shown in FIG. 1, a plurality of cooling air passages are formed on the cooling plate, and each of which branches as it extends toward the cooling air outlet. That is to say, the width of the cooling air passage (the area on the cell having contact with the cooling air) is narrow at the inlet and readily becomes wider as it extends towards the outlet, so that the fuel cell will be cooled less at the inlet and more at the outlet.
However, this method is not effective in terms of efficiency in generating electricity for the following reason. Narrowing the width of the cooling air passage at the inlet causes a considerable pressure loss, and the pressure must be increased to fully supply the cooling air throughout the branched passages to the outlet, Since the fuel cell uses self-generated electricity to supply the cooling air, the more power used to supply the cooling air, the less the efficiency becomes in electricity generation.